In general, when a man breaks a bone or dislocates a bone, the man undergoes a reduction treatment so as to have the bone treated. In the case of conducting a reduction treatment, a person conducting the reduction treatment such as a doctor or a reduction practitioner makes a patient's leg or the like perform various operations such as extending or contracting, bending, or twisting by the use of his own power. However, to make the patient's leg or the like perform the various actions requires a considerable amount of power, which results in imposing a heavy physical labor on the doctor or the reduction practitioner.
Thus, in order to solve this problem, a reduction apparatus is used. In this reduction apparatus is performed a power assist control for assisting force in a direction in which the force is applied. In other words, in the power assist control in a bone fracture reduction apparatus, when a man applies force, a motor is driven in a direction to assist the force (power).
In the related art, there has been employed a method for assisting force applied by a man, which is obtained by multiplying force by a constant, by a motor, that is, a method for outputting a linear force proportional to force applied by a man (see Patent Document 1)    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-348699    Patent Document 2: Japanese Patent Application Laid-Open No. 2003-252600    Patent Document 3: Japanese Patent No. 3188953